In optical communication or measurement of a time of flight (TOF), an avalanche photodiode utilizing an avalanche amplification (avalanche) effect of a photodiode has been conventionally used as a light receiving element that detects weak light at high speed (for example, refer to PTL 1). When a reverse bias voltage less than a breakdown voltage is applied, the avalanche photodiode operates in a linear mode and an output current varies so as to have a positive correlation with a light receiving amount. On the other hand, when a reverse bias voltage equal to or more than the breakdown voltage is applied, the avalanche photodiode operates in a Geiger mode. The avalanche photodiode in the Geiger mode causes an avalanche phenomenon even in a case of incidence of a single photon, so that a large output current is obtained. Thus, the avalanche photodiode in the Geiger mode is called a single photon avalanche diode (SPAD).
When a quenching resistance is applied in series to the avalanche photodiode in the Geiger mode, a binary pulse output is able to be obtained. Such a circuit is constituted by, for example, a photodiode, an active quenching resistance (resistance component of a MOS transistor), and a buffer.
The aforementioned photodiode is an avalanche photodiode in the Geiger mode, and, when being applied with the bias voltage equal to or greater than the breakdown voltage, causes an avalanche phenomenon for incidence of a single photon, so that a current flows. When the current flows through the aforementioned active quenching resistance connected in series to the photodiode, a voltage between terminals of the active quenching resistance increases, and the bias voltage of the photodiode drops accordingly, so that the avalanche phenomenon stops. When there is no current by the avalanche phenomenon, the voltage between the terminals of the active quenching resistance decreases, and the photodiode returns to a state of being applied with the bias voltage equal to or greater than the breakdown voltage again. By the aforementioned buffer, the change of the voltage between the photodiode and the active quenching resistance is extracted as a binary pulse output.
Moreover, PTL 2 discloses a method of measuring a distance in such a manner that reflection light and direct light from a light emitting element are input to different delay locked loop circuits (DLLs) with use of the aforementioned SPAD and a delay amount between outputs of the two DLLs is converted into a digital value.